1. Field of the Invention
The present invention relates to on-demand type method and apparatus for discharging liquid, and particularly, to a method and an apparatus or a head for discharging liquid which are capable of using satellite droplet groups in the same manner as a main droplet in an appropriately discharged state, and which achieve a high-speed and high-frequency discharge. The present invention further relates to a liquid discharging method, an image forming apparatus, a liquid discharge apparatus, and a liquid discharge head which can be used for the entire spectrum of the technical field including printers which uses various liquids by bringing them into minute droplets.
2. Description of the Related Art
Hitherto, conventional arts of forming droplets have been broadly classified into the continuous discharge system and the on-demand system. As disclosed in Japanese Examined Patent Application Publication No. 6-24871, the former system pressurizes liquid into a high pressure and brings a discharged liquid column into minute droplets by means of electrolysis, and employs a large head which separates the droplets from electrically charged liquid by an electric field control. On the other hand, the latter system uses a small head which discharges droplet-shaped liquids in a drive timing by means of an electrothermal transducer, an electromechanical transducer, or the like.
In recent years, in the on-demand system, high-quality images has been developed for commercial use by the techniques of bringing color droplets into minute droplets. Among these techniques for bringing color droplets into minute droplets, there is a method which is applied by the same assignee as this application and in which a bubble is made to communicate with atmosphere, a method in which merely the size-reduction of a discharge port is conducted. To say nothing of these techniques of bringing color droplets into minute droplets, in the droplet discharging method, there occurs a plurality of minute droplets, named “satellite droplets”, which are smaller in the size and lower in the speed than a main droplet. This is because a reduced speed distribution occurs with respect to the speed of the main droplet, the reduced speed distribution being caused by the action force in the direction opposite to the direction of the liquid movement inward to the head side due to the recession of a meniscus resulting from the contraction of a bubble and the deformation of a piezoelectric element (see e.g. Japanese Examined Patent Application Publication No. 59-31944).
FIGS. 2A to 2C illustrate states of droplet discharging resulting from the occurrence of this speed distribution. In the figures, a main droplet is discharged at a speed V1′, and a group of smaller satellite droplets are gradually brought into minute droplets, so that the speed thereof become V2, V3, V4, V5, and V6 in descending order (V1′>V2>V3>V4>V5>V6). A droplet having a speed from V4 onward, comes into mist constituted of minute droplets, as described later.
Japanese Laid-Open Patent Applications Nos. 9-1790 and 10-193649, which are applied based on a concept of using the above-described satellite droplets for image forming, in a state individually separated, with respect to the main droplet, disclose the technique which makes variable the dot area to be formed by bringing the satellite droplets close to the main droplet by controlling a drive pulse to an electromechanical transducer for bubble forming. Also, Japanese Laid-Open Patent Applications No. 7-285222, which addresses the problem of satellite droplets that differs from the above-described satellite droplets, and that are obtained by two-time discharges peculiar to a piezoelectric element, discloses a concept that a satellite droplet which is generated by the projection of a meniscus to the outside of the discharge port due to a large residual vibration of the meniscus accompanied with a rebound displacement of an electromechanical transducer (piezoelectric element) after the deformation thereof, is discharged with a weight equal to that of the main droplet provided, and that the satellite droplet is coalesced with the main droplet on a medium so as to overlap the main droplet. However, in this Japanese Laid-Open Patent Applications No. 7-285222, since there is no perception that new satellite droplets further occur after the discharged satellite droplet with a weight equal to that of the main droplet provided, a fundamental solution of satellite droplets has not yet attained. This is because, since a second meniscus projection accompanied with the rebound displacement of the piezoelectric element is utilized, even if a voltage application for a drive is performed only one time, it will be eventually equivalent to performing discharge two times because the piezoelectric type discharge head discharges the liquid of an equal weight. The displacement of the meniscus, therefore, continues even after the second discharge, as a natural result, so that a further satellite droplet will occur.
Minute droplets, which is named mist, and each of which is more minute than a droplet, such as satellite droplet, having a velocity component and a quantity enough to adhere to a paper surface tend to increase in the number as the main droplet decreases in the quantity. As countermeasures against this, only a technical development for removing occurred mists is being conducted. Japanese Examined Patent Application Publication No. 5-57913, and the corresponding U.S. Pat. No. 468,539, which recognize this mist issue, discloses that the problem of the occurrence of a plurality of satellite droplets with respect to a main droplet can be solved by a method wherein all satellite droplets are coalesced with the main droplet by performing first, second, and third pulse drives for a piezoelectric element. The Japanese Examined Patent Application Publication No. 5-57913, does not analyze the mechanism that the satellite drops are coalesced with the main droplet is not analyzed, but discloses a technical concept of allowing the satellite droplets to be coalesced with the main droplet in a space. Also, in U.S. Pat. No. 4,491,851, the condition for preventing the occurrence of satellite droplets by performing first and second pulse drives for a piezoelectric element as in the case of the above patents, and the condition that, even if the satellite droplets are generated, satellite droplets are coalesced with a main drop as being in a high-speed satellite-dot area condition, are disclosed in FIG. 9 relative to the conventional satellite-droplet generation area condition. The above-described patents shows the results wherein satellite droplets are coalesced with a main droplet in midair, but, since each of these results has been acquired by performing plural-time pulse drives for a piezoelectric element, each of these methods is not practical for performing a high-speed recording or a high-frequency discharging because of an elongated driving time. On the other hand, Japanese Patent Application No. 2000-227081, which proposes means for coalescing a satellite droplet with a main droplet by a discharging method using an electrothermal transducer which forms a bubble, discloses a method wherein a satellite droplet is coalesced with a main droplet by increasing the speed of the satellite droplet by a movable member displaced by one-time bubble forming.
The technical level of the conventional arts recognized from the foregoing is such that plural-time displacements for discharging satellite droplets are performed with respect to a piezoelectric element. The mainstream of the conventional arts is the technique of integrating occurred satellite droplets into a main drop in response to the driving conditions for providing the above-described plural-time deformations to the piezoelectric element. However, the plural-time deformations for discharge with respect to the piezoelectric element runs counter to the increasing of the driving frequency, and delays the return of the meniscus after discharging to the steady state. Therefore, this counteracts the improvement in the print speed by a high-speed and high-frequency drive. In addition, rebound displacements occur after the plural-time displacements of the piezoelectric element, as a natural result, thereby generating further satellite droplets. This constitutes a dilemmatical problem. Particularly, in the Japanese Laid-Open Patent Applications No. 7-285222, since the piezoelectric element is given a drive such as to make the satellite droplet one having an equal quantity as the main droplet which has discharged the satellite droplet, the same satellite droplet problem as conventional one, that is, a dilemmatical problem incapable of solution arises.
Judging from the foregoing, the conventional arts are only at a technical level at which satellite droplets are used so as to eventually enlarge the shot image of a main droplet when viewing the image, since the image is formed with satellite droplets coalesced with or brought close to the main droplet.
Anyhow, at the level of the conventional arts, there is provided no solution as to how to position a satellite drop group relative to a main drop in order to perform a high-speed discharge or a high-pressure discharge.
From another viewpoint, no technical attention has been directed toward a technical problem as to how mists, i.e., minute satellite droplets are prevented from occurring in order to obtain a high-quality color image formed by high-speed and minute droplets, and no solution to this technical problem has been proposed.